Voltage converter for implantable microstimulator using RF-powering coil

ABSTRACT

A combination, voltage converter circuit for use within an implantable device, such as a microstimulator, uses a coil, instead of capacitors, to provide a voltage step up and step down conversion functions. The output voltage is controlled, or adjusted, through duty-cycle modulation. In accordance with one aspect of the invention, applicable to implantable devices having an existing RF coil through which primary or charging power is provided, the existing RF coil is used in a time-multiplexing scheme to provide both the receipt of the RF signal and the voltage conversion function. This minimizes the number of components needed within the device, and thus allows the device to be packaged in a smaller housing or frees up additional space within an existing housing for other circuit components. In accordance with another aspect of the invention, the voltage up/down converter circuit is controlled by a pulse width modulation (PWM) low power control circuit. Such operation allows high efficiencies over a wide range of output voltages and current loads.

[0001] The present application is a continuation application of U.S.application Ser. No. 09/799,467, filed Mar. 5, 2001, which applicationclaims the benefit of Provisional Application Serial No. 60/189,992,filed Mar. 17, 2000. Both applications are herein incorporated byreference in their entireties.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to implantable medical devices, andmore particularly to a voltage converter for use within an implantablemicrostimulator, or similar implantable device, that uses an RF-poweringcoil instead of capacitors to provide a voltage step-up and step-downfunction.

[0003] Many implantable medical devices, such as neural stimulators,sensors, and the like, utilize a battery as a primary source ofoperating power. Other types of implantable devices, such as cochlearstimulators, rely on the presence of an alternating magnetic field toinduce an ac voltage into the implantable device, where the inducedvoltage is thereafter rectified and filtered in order to provide theprimary operating power for the device. In both types of devices—abattery-powered device or an RF-powered device—there is a frequent needto derive other operating voltages within the device from the primarypower source. That is, there is a frequent need to step up the voltageof the primary power source to a higher voltage in order to, e.g.,generate a high stimulation current or for some other purpose.Similarly, in some devices, there is also a frequent need to step downthe voltage of the primary power source to a lower voltage for use incertain types of circuits in order to, e.g., conserve power.

[0004] In order to perform the voltage step-up or step-down function, itis known in the art to use a charge-pump voltage converter circuit.Charge pump circuits typically rely on a network of capacitors andswitches in order to step up and step down a primary voltage source. Forexample, in order to step up a primary voltage source, a network of,e.g., four capacitors, may be connected in parallel through a switchingnetwork and maintained in the parallel connection configuration untileach capacitor charges to the voltage of the primary power source. Thevoltage of the primary power source is, e.g., the battery voltage (wherea battery is used as the primary power source). Once thus charged, thecapacitors are switched so that they are connected in series, therebyeffectively creating a voltage across the series connection that is fourtimes the voltage of the primary voltage source. The charge associatedwith this higher voltage may then be transferred to another capacitor,e.g., a holding capacitor, and this process (or chargingparallel-connected capacitors, switching them in series, and thentransferring the charge from the series connection to a holdingcapacitor) is repeated as many times as is necessary in order to pump upthe charge on the holding capacitor to a voltage that is four times asgreat as the voltage of the primary power source.

[0005] While charge-pump circuits have proven effective for performingstep up and step down functions, such circuits require a large number ofcapacitors, which capacitors may be quite large and bulky. Charge pumpcircuits that use large numbers of bulky capacitors are not well suitedfor implantable medical devices that must remain very small. Moreover,charge pump circuits tend to be relatively slow and inefficient inoperation. What is needed, therefore, is a voltage converter circuitthat is able to perform the step up or step down function, efficiently,quickly, and without having to rely on the use of a large number ofbulky capacitor/s.

SUMMARY OF THE INVENTION

[0006] The present invention addresses the above and other needs byproviding a voltage converter for use within small implantableelectrical devices, such as a microstimulator, that uses a coil, insteadof capacitors, to provide the voltage step up and step down function.The output voltage of such converter is controlled, or adjusted, throughduty-cycle and/or ON/OFF modulation. Hence, good efficiencies areachieved for virtually any voltage within the compliance range of theconverter.

[0007] In accordance with one aspect of the invention, applicable toimplantable devices having an existing RF coil through which primary orcharging power is provided, the existing RF coil is used in atime-multiplexing scheme to provide both the receipt of the RF signaland the voltage conversion function. This minimizes the number ofcomponents needed within the device, and thus allows the device to bepackaged in a smaller housing, or frees up additional space within anexisting housing for other circuit components. The result is animplantable device having a voltage converter that may be much smallerand/or more densely packed than prior implantable devices.

[0008] In accordance with another aspect of the invention, the voltageup/down converter circuit is controlled by a pulse width modulation(PWM) and/or ON/OFF modulation (OOM) low power control circuit. Suchoperation advantageously allows high efficiencies over a wide range ofoutput voltages and current loads.

[0009] According to another aspect of the invention, an implantabledevice containing a coil is provided, wherein the coil is used formultiple purposes, e.g., for receiving power from an external source andalso as part of a voltage conversion circuit. Alternatively, orconjunctively, the coil may be used for receiving command informationfrom an external source and also as part of a voltage conversioncircuit.

[0010] It is thus a feature of the present invention to provide avoltage converter circuit for use within an implantable device, e.g.,such as an implantable microstimulator or similar type of neuralstimulator, that is compact, efficient, and provides a wide range ofoutput voltages and currents.

[0011] It is a further feature of the invention to provide a voltageconverter circuit that avoids the use of a network of capacitorsswitched between parallel and series, or other, configurations in orderto provide the step up and step down voltage conversion function.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other aspects, features and advantages of thepresent invention will be more apparent from the following moreparticular description thereof, presented in conjunction with thefollowing drawings wherein:

[0013]FIG. 1 is a block diagram of an implantable stimulator system;

[0014]FIG. 2 is a sectional schematic diagram that illustrates one typeof implantable microstimulator within which the present invention may beused;

[0015]FIG. 3 is a functional block diagram of a typical implantablestimulator;

[0016]FIG. 4 illustrates a type of fly back converter circuit that maybe used to step up the voltage of a power source without the use of aswitched capacitor network;

[0017]FIG. 5 is a waveform diagram that defines what is meant by “dutycycle” for purposes of the present application;

[0018] FIGS. 6A-6E illustrate simplified schematic diagrams of circuitsthat may be used in accordance with the present invention torespectively achieve the following implantable-device functions: voltagestep up (FIG. 6A); voltage step down (FIG. 6B); energy reception (FIG.6C); data reception (FIG. 6D); and data transmission (FIG. 6E);

[0019]FIG. 7 is a simplified schematic diagram that illustrates avoltage converter circuit made in accordance with the present inventionthat selectively performs the five implantable-device functionsillustrated in FIGS. 6A-6E; and

[0020]FIG. 8 is a table that defines the operating state of the variousswitches M1′, M2, M3, M4 and M5 utilized in the circuit of FIG. 7 inorder to select a desired operating mode for the circuit shown in FIG.7.

[0021] Corresponding reference characters indicate correspondingcomponents throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The following description is of the best mode presentlycontemplated for carrying out the invention. This description is not tobe taken in a limiting sense, but is made merely for the purpose ofdescribing the general principles of the invention. The scope of theinvention should be determined with reference to the claims.

[0023] The present invention relates to a particular type of voltageconverter that may be used within an implantable medical device, such asan implantable stimulator, sensor, pump or other type of medical deviceproviding a desired medical function. The invention will be describedbelow in terms of an implantable stimulator, but it is to be understoodthat the invention may be used within many different types ofimplantable devices.

[0024] To better understand the environment in which the invention isintended to be used, it will first be helpful to review a typicalimplantable stimulation system. Hence, with reference to FIG. 1, a blockdiagram of a representative implantable stimulator system 10 isillustrated. The system 10 includes an implant device 20, implantedunder the skin 18, coupled to an external control unit 12 throughimplanted coil 22 and external coil 15. The external coil 15 istypically carried in a housing 14 connected to the external control unit12 via flexible cable 13. An external power source 16, which may be,e.g., a rechargeable or replaceable battery, provides operating powerfor the external control unit. The external power source 16 may alsoprovide operating power for the implant device 20 through the linkprovided through the coils 15 and 22, either continuously or on anintermittent basis. Intermittent power is provided, e.g., such as whenthe implant device includes a replenishable power source, such as arechargeable battery, and the battery is intermittently recharged.

[0025] The implant device 20, when functioning as a stimulator, includesa plurality of electrodes 24 a and 24 b connected to the implant device20 via conductive leads or wires 23 a and 23 b, respectively. Theelectrodes 24 a and 24 b are typically implanted near body tissue ornerves 26 that are to be stimulated.

[0026] In operation, the system 10 functions as follows: The implantdevice 20 and electrodes 24 a and 24 b are implanted in the desiredlocation under the patient's skin 18. It should be noted that while theimplant coil 22 is shown separate from the implant device 20 in FIG. 1,the coil 22 is typically mounted to or housed within the samehermetically-sealed case used to house the electronic circuitryassociated with the implant device 20. Once implanted, power and/orcontrol data, e.g., programming data, is transferred to the implantdevice from the external control unit 12 via electromagnetic couplingbetween the implant coil 22 and the external coil 15. Once thuscontrolled or programmed, the implant device 20 operates as directed bythe control signals received, or as steered by the program data storedtherein, to generate electrical stimulation pulses for delivery to thetissue 26 via the electrodes 24 a and 24 b.

[0027] Some implant devices 20 do not contain an implanted power source,and such devices must thus receive their operating power continuouslyfrom the external control unit. Other implant devices 20 do contain animplanted power source, e.g., a rechargeable battery, and such devicesthus receive their operating power from the implanted power source.However, on a regular or periodic basis, such devices must have theimplanted power source replenished, e.g., have the battery recharged.Such recharging occurs via a link with the external control unit 12, orequivalent device, through the coils 22 and 15.

[0028]FIG. 2 shows a sectional schematic diagram of one type ofimplantable microstimulator 30 within which the present invention may beused. The microstimulator device 30 includes electrical circuitry 32housed within a hermetically-sealed case 34. At each end of the case 34are electrodes 36 a and 36 b. These electrodes 36 a and 36 b areelectrically connected to the electrical circuitry 32 via conductors,e.g., wires, 37 a and 37 b, respectively, and appropriate feed-throughconductors 38 a and 38 b that pass through the wall of thehermetically-sealed case 34.

[0029] The advantage of the microstimulator device 30 is that it is verysmall, and can typically be easily implanted at the desired implantlocation through the lumen of a hypodermic needle, or other cannula. Oneembodiment of a microstimulator is disclosed, e.g., in U.S. Pat. No.5,324,316, incorporated herein by reference. One method of making such amicrostimulator is disclosed, e.g., in U.S. Pat. No. 5,405,367, alsoincorporated herein by reference.

[0030] To better appreciate the advantages offered by the presentinvention, reference is next made to FIG. 3 where there is shown afunctional block diagram of a typical implantable stimulator 40. As seenin FIG. 3, the stimulator 40 includes electronic circuitry that performsthe following functions: an energy receiver 42, a data receiver 44, apower source 46, a control circuit 48, a voltage converter 50, a pulsegenerator 52, and a back telemetry circuit 54. An implanted coil 56 isconnected to both the energy receiver 42 and the data receiver 44 andprovides a means through which power and data signals may be received bythe stimulator 40. Another coil 58, which in some embodiments maycomprise the same, or a portion of, the coil 56, is connected to theback telemetry circuit 54, and provides a means through which backtelemetry data may be sent to an external receiver. Such an externalreceiver may be included, for example, within the external control unit12 (FIG. 1). All of the above-described elements of the stimulator 40are housed within an hermetically-sealed housing or case 60, therebyallowing the stimulator 40 to be implanted within body tissue.

[0031] External to the housing 60, but still adapted to be implantedwithin body tissue, is a plurality of electrodes 62 a, 62 b. Electricalconnection with the plurality of electrodes 62 a, 62 b is establishedthrough a plurality of wire conductors 63 a, 63 b (which may be includedwithin a single implantable lead body, as is known in the art) which arerespectively connected to a plurality of feed-through connectors 64 a,64 b that pass through the hermetically-sealed wall of the case 60. Thepulse generator 52 is electrically coupled to the plurality offeed-through connectors 64 a, 64 b on the inside of the case 60.

[0032] In operation, an RF signal (represented in FIG. 3 by the wavyarrow 66) is received through coil 56. Typically, the RF signalcomprises a modulated carrier signal. The carrier signal is rectified inthe energy receiver 42 and provides charging power for the power source46. The carrier signal is demodulated in the data receiver 44 and thedata thus recovered provides control and/or programming data to thecontrol circuit 48. The control circuit 48, typically a microprocessor,includes memory circuitry (not shown) wherein programming and/or controldata may be stored. Based on this programming and/or control data, thecontrol circuit 48 drives the pulse generator circuit 52 so that itgenerates and delivers electrical stimulation pulses to the patientthrough selected groupings of the plurality of electrodes 62 a, 62 b.

[0033] In the process of generating the electrical stimulation pulses,which typically vary in amplitude as a function of the control and/orprogramming data, and in order to conserve power, it is necessary toprovide a high level supply voltage to the pulse generator circuit 52.For example, if the impedance between electrodes 62 a and 62 b is 1000ohms, and if a stimulation current pulse having a magnitude of 10 ma isdesired, a voltage of 10 volts must be present at the electrodes 62 aand 62 b (Ohms law: voltage=current x impedance). This means that anoutput voltage V_(O) of at least 10 volts must be present at the outputof the pulse generator circuit 52. In turn, this means that a supplyvoltage V_(C), provided to the pulse generator circuit by the voltageconverter 50, must be greater than 10 volts, e.g., 12 volts or more dueto losses within the pulse generation circuit. Hence, the voltageconverter circuit 50 is typically used in a stimulator 40 to step up thepower source voltage V_(S), e.g., the battery voltage, to a levelsuitable for use by the pulse generator circuit 52. The power sourcevoltage V_(S) is typically a low value, e.g., 2 or 3 volts. Hence, in atypical stimulator device 40, such as the one shown in FIG. 3, thevoltage converter circuit 50 is needed to boost, or step up, the sourcevoltage V_(S) from its relatively low value to a higher level V_(C) asneeded by the pulse generator circuit 52. Unfortunately, in order toprovide such a step-up function, bulky and numerous circuit components,such as the capacitors used in a switched capacitor network, and/ortransformers, must be employed.

[0034] The difference between the supply voltage V_(C) and the outputvoltage V_(O) may be referred to as the compliance voltage. In an idealpulse generator circuit 52, the compliance voltage is kept as low aspossible because the power dissipated in the pulse generator circuit(which is generally considered as wasted or lost power because it doesnot represent power delivered to the tissue) is proportional to thesquare of the compliance voltage. In practice, the compliance voltagecannot always be minimized because the current delivered through theelectrodes 62 a and 62 b to the body tissue varies over a wide range;and hence the compliance voltage must also vary over a wide range.

[0035] In some implantable stimulators 40, in order to conserve theamount of power dissipated by the stimulator, the voltage convertercircuit 50 is used to adjust the supply voltage V_(C), typically toprovide a small number of discrete levels of supply voltage, as afunction of the current to be delivered in the stimulation pulse. Forexample, a typical voltage converter circuit 50 may provide one of fourdifferent supply voltages V_(C) to the pulse generator circuit 52, e.g.,a V_(C) of 2.5, 5.0, 7.5 or 10 volts, as a function of the programmedamplitude of the stimulation pulse that is to be delivered to thetissue. An implantable stimulator having such a feature is described,e.g., in U.S. Pat. No. 5,522,865, incorporated herein by reference.

[0036] It is thus seen that the voltage converter circuit 50 performs avery important function within the implantable stimulator 40.Unfortunately, however, the voltage converter circuit 50 representsadditional circuitry that requires bulky circuit components, which takesup needed and valuable space within the case 60, and much of which alsoconsumes additional power. Further, most voltage converter circuits 50tend to be very inefficient. That is, a capacitor charge pump circuit,for example, typically may operate at efficiencies that may be less than50%. Thus, for most stimulators, e.g., of the type shown in FIG. 2,space and power considerations are paramount to the design of thestimulator.

[0037] The present invention advantageously provides circuitry for usewithin an implantable stimulator device that performs the voltageconversion function using fewer and less bulky components. This frees upvaluable space within the case of the stimulator that may be used forother functions (or allows the case to be smaller), and consumes lesspower than has heretofore been achievable. Additionally, the presentinvention provides a circuit that performs multiple functions, thusallowing fewer circuit components to be used within the stimulatordesign, thereby permitting the overall stimulator design to be smalleror more compact.

[0038] Turning next to FIG. 4, a type of fly back converter circuit isillustrated that may be used to step up the voltage of a power sourcewithout the need for a switched capacitor network. The fly back circuitshown in FIG. 4 includes an inductor or coil L1 having one end connectedto a power source 70. The other end of the coil L1 is connected to afirst circuit node 72. A switching transistor M1 is connected betweenthe first node 72 and ground. The transistor M1 has a gate terminal 73connected to a duty cycle control circuit 74. When the transistor M1 isturned ON, through application of a signal to its gate terminal 73, node72 is effectively switched to ground potential through a very lowimpedance path. When transistor M1 is turned OFF, through absence of asignal applied to its gate terminal 73, it represents a very highimpedance path, and thus effectively maintains node 72 disconnected fromground.

[0039] Also connected to node 72 of the fly back circuit shown in FIG. 4is the cathode side of diode D1. The anode side of diode D1 is connectedto an output node 75. An output capacitor C1 is connected between theoutput node 75 and ground. A load, represented in FIG. 4 by phantomresistor RL, is also connected between the output node 75 and ground.

[0040] Still with reference to FIG. 4, the duty cycle control circuit 74applies a pulsed signal to the gate of transistor M1, therebyeffectively turning transistor M1 ON and OFF as controlled by the pulsedsignal. For example, a high voltage applied to the gate of M1 may turnM1 ON (provide a low impedance path between node 72 and ground), and alow voltage applied to the gate of M1 may turn M1 OFF (provide a highimpedance path between node 72 and ground). A sequence of high and lowvoltages may be applied to the gate 73 of transistor M1 throughapplication of a pulsed signal 81 generated by the duty cycle controlcircuit 74. When a pulse is present, the voltage is high, and thetransistor M1 is turned ON. When a pulse is not present, the voltage islow, and the transistor Ml is turned OFF.

[0041] The ratio of time when the pulse is high to the total cycle timeis known as the “duty cycle”. The duty cycle is defined as shown in FIG.5. As seen in FIG. 5, a pulsed signal 81 comprises a train of pulses 80.Each pulse 80 comprises a high voltage for a period of time T2 and a lowvoltage for a period of time T3. The total cycle time T1 is equal to T2plus T3. Duty cycle is typically defined as a percentage and is computedas the ratio of T2/T1 or T2/(T2+T3). The duty cycle may thus vary from0% when T1=0, to 100% when T1=T2.

[0042] The operation of the fly back circuit of FIG. 4 is known in theart. Basically, when transistor M1 is turned ON, during time period T2,circuit node 72 is connected to ground, which connects one side of thecoil L1 to ground. This connection of one side of the coil L1 to groundcauses an electrical current to start to flow from the power source 70through the inductor coil L1. As soon as T2 ends, however, and for theremaining time T3 of the total cycle time T1, the node 72 floats (is notconnected to ground), which causes the voltage at node 72 to step up toa high value (higher than the voltage of the power source Vs, aselectrical current continues to flow through coil L1, through diode D1,to charge capacitor C1. Thus, during time T2, current starts to flowthrough the coil L1, which causes electromagnetic energy to be stored inthe coil. During time T3, this energy is transferred to capacitor C1,thus charging C1. Eventually, typically over several cycles, C1 ischarged up to a voltage that is higher than the power source voltage Vs.Capacitor C1 is blocked from discharging to ground through transistor M1by diode D1 when M1 is turned ON during time T2. The stored charge heldon capacitor C1 thus provides an output voltage VOUT (greater thanV_(S)) that causes an output current I_(O) to flow through the loadresistor R_(L).

[0043] The magnitude of the output voltage V_(OUT) and output currentI_(O) may advantageously be controlled by adjusting the duty cycle ofthe signal 81. A higher duty cycle causes both V_(OUT) and I_(O) toincrease, whereas a lower duty cycle causes V_(OUT) and I_(O) todecrease. Because the duty cycle is adjusted by controlling the pulsewidth (T2) of the pulses 80, the duty cycle control circuit 74 may alsobe referred to as a pulse width modulator circuit.

[0044] Still with reference to FIG. 4, it should also be noted thatfeedback may optionally be employed to better control and regulate theoutput voltage V_(OUT). That is, a sensing circuit 76A may be used tomonitor the output voltage V_(OUT), and to compare the sensed outputvoltage to either a reference voltage V_(REF) and/or a programmedreference signal PROG (which typically is presented to the sensingcircuit 76A as a digital signal). The sensing circuit 76A generates adifference signal, on signal line 76C, representing the differencebetween the sensed output voltage V_(OUT) and the reference voltageV_(REF) and/or PROG. This difference signal controls a gate controlcircuit 76B, which modulates the gate of transistor M1 so as to drivethe difference signal to zero.

[0045] Turning next to FIGS. 6A-6E, additional simplified schematicdiagrams of circuits are illustrated that may be used in accordance withthe present invention to achieve desired functions. More particularly, avoltage step up function may be achieved using the circuit shown in FIG.6A; a voltage step down function may be achieved using the circuit ofFIG. 6B; an energy reception function may be achieved using the circuitof FIG. 6C; a data reception function may be achieved using the circuitof FIG. 6D; and a data transmission function may be achieved using thecircuit of FIG. 6E. Advantageously, many of the components used in thecircuits of FIGS. 6A-6E may be the same. Common reference numerals areused to denote the components that may be the same. A brief explanationof each of these functions will next be described.

[0046]FIG. 6A depicts a circuit that performs a voltage step upfunction. This circuit is substantially the same as the circuitpreviously described in connection with FIG. 4, except that the loadresistance R_(L) is not shown. However, it is to be understood that aload resistance may be present. It should also be understood thatwhereas FIG. 4 shows a duty cycle control circuit 74 controlling switchM1, FIG. 6A shows a PWM (pulse width modulation) control circuit 74′controlling switch M1. These circuits perform the same function (turningswitch M1 ON or OFF) and, for purposes of the present invention, aresubstantially the same.

[0047]FIG. 6B depicts a circuit that performs a voltage step downfunction. As seen in FIG. 6B, a coil L1 is connected between circuitnodes 75 and 76. Node 75 represents the output node of the circuitwhereon the output voltage V_(OUT) is present. Capacitor C1 is connectedbetween node 75 and ground. The anode side of a diode D2 is connected tonode 76, while the cathode side of diode D2 is connected to ground. Oneleg of a transistor switch M2 is connected to node 76, while the otherleg of transistor switch M2 is connected to the power source 70 at node77. A gate terminal 78 of transistor M2 is connected to pulse-widthmodulation (PWM) control circuit 74″.

[0048]FIG. 6C shows a circuit that receives energy from an externalsource. The energy receive circuit shown in FIG. 6C includes a coil L1having a capacitor C1 connected in parallel with the coil L1, with oneside of the parallel connection being grounded. The coil L1 andcapacitor C1 comprise an “LC” circuit that is tuned to the frequency ofan incoming RF signal 83 (represented in FIG. 6C by a wavy arrow). DiodeD1 is connected between output node 75 and the other side of the L1-C2parallel connection, with the cathode of D1 being connected to node 75.Capacitor C1 is connected between output node 75 and ground.

[0049] In operation, the circuit shown in FIG. 6C receives the incomingRF signal 83 through coil L1, tuned to the frequency of the signal 83 bycapacitor C2. Diode D1 rectifies the signal, storing the positive halfcycles of the received signal 83 on capacitor C1. The voltage thusdeveloped on capacitor C1 functions as an output voltage VOUT for usewithin the implant device.

[0050] Next, in FIG. 6D, a data receiver circuit is illustrated. Suchdata receiver circuit includes coil L1 connected in parallel withvariable capacitor C3. A modulated RF signal 88′ is received through thecoil L1. The value of C3 is adjusted, as required, so that the L1-C3circuit is tuned to the frequency of modulation applied to the incomingRF signal 88′. Node 72′, which represents an output node of the L1-C3circuit, is connected to the input of an amplifier U1. The output signalprovided by the amplifier U1 comprises a Data Out signal that reflectsthe modulation applied to the incoming modulated RF signal 88′.

[0051] Turning to FIG. 6E, a simple data transmitter circuit isdepicted. The data transmitter circuit includes a coil L1 connected inparallel with an adjustable variable capacitor C3. One side of the L1-C3parallel connection is connected to a power source 70. The other side ofthe L1-C3 parallel connection, identified as node 72′ in FIG. 6E, isconnected to the anode of diode D3. The cathode of diode D3 is connectedthrough a switch transistor M3 to ground. The gate terminal of switch M3is driven by a “Data Mod” (data modulation) signal. Thus, in operation,when switch M3 is closed, a current is drawn through the L1-C3 parallelcircuit. When switch M3 is open, no current is drawn through the L1-C3parallel connection. The on-off current flow through the L1-C3 parallelconnection causes a varying current to flow through coil L1 ascontrolled by the on-off pattern of the Data Mod signal. This currentflow, as is known in the art, induces a varying magnetic field, which inturn causes an RF signal 89 to be radiated, or transmitted, from coilL1.

[0052] Thus it is seen that the circuits illustrated in FIGS. 6A-6Eprovide the functions of voltage step up (FIG. 6A), voltage step down(FIG. 6B), energy reception (FIG. 6C), data reception (FIG. 6D), anddata transmission (FIG. 6E). All of these functions are typicallyrequired within an implantable stimulator device (FIG. 3).

[0053] In order to perform the functions provided by the circuits shownin FIGS. 6A-6E, while at the same time reducing the number of componentsneeded for each function, and thereby reduce the overall size (and hencevolume, weight and power) of the circuitry that carries out suchfunctions, the present invention advantageously combines all thefunctions performed by the individual circuits shown in FIGS. 6A-6E intoone circuit as shown in FIG. 7. Such combined circuit may be referred toas a “voltage converter using an RF-powering coil”, and is particularlysuited for use within an implantable medical device, such as animplantable neural stimulator.

[0054] Advantageously, the combined circuit provided by the presentinvention, and shown in FIG. 7, uses an RF-powering coil in combinationwith other circuit elements to perform the function of receiving RFpower from an external source. The received RF power may be modulated inorder to transmit control data into the circuit. Further, suchRF-powering coil may be used to help transmit data out of the circuit.Significantly, the RF-coil used to receive power, data, and to transmitdata, may also be used to selectively convert the received power (i.e.,voltage) up or down in order to make operation of the circuit moreefficient.

[0055] The circuit of FIG. 7 (i.e., the voltage converter circuit usingan RFpowering coil provided by the present invention) includes areceiving/transmitting coil L1′. The coil L1′ includes ends attached tocircuit nodes 72′ and 85, respectively. Node 85, in turn, is connectedthrough transistor switch M1′ to source voltage V_(S). The coil L1′further includes a tap point 85′, where there are N2 turns of the coilbetween tap point 85′ and node 85, and N1 turns between tap point 85′and node 72′. The coil L1′ thus has a total of N turns, where N=N1+N2.Representative values of N1 are 10 to 100 turns, and for N2 are also 10to 100 turns, and wherein the inductance of coil L1′ is between about 10to 100 microhenries (pH). However, in some embodiments, N1 and N2 mayvary from 1 to 1000 turns, and L1′ may vary between 1 to 1000 pH.

[0056] Still with reference to FIG. 7, a series combination of acapacitor C3′ and transistor switch M4 is connected between circuit node72′ and tap point 85′. Another transistor switch M5 connects the tappoint 85′ of coil L1′ to ground (node 87). Yet another transistor switchM2 connects the tap point 85′ to the source voltage V_(S).

[0057] The cathode end of a diode D2 is also connected to the tap point85′ of the coil L1′; while the anode end of diode D2 is connected toground.

[0058] The cathode end of another diode D3 is connected to node 72′. Theanode end of diode D3 is connected through transistor switch M3 toground (node 87). The anode end of diode D3 is also connected to theinput of signal amplifier U1.

[0059] The cathode end of yet another diode D1 is also connected to node72′. The anode end of diode D1 is connected to circuit node 75′. Acapacitor C1 is connected between node 75′ and ground (node 87). Circuitnode 75′ is the location where the output voltage V_(OUT) is madeavailable when the circuit operates in a voltage step up or step downmode. If needed, a suitable voltage clamp circuit 91 may be connectedbetween node 75′ and ground in order to prevent the voltage at theoutput node 75′ from exceeding some predetermined value.

[0060] It is thus seen that the circuit of FIG. 7 includes fivetransistor switches, M1′, M2, M3, M4 and M5. The state of these fiveswitches, whether ON, OFF, or modulated with PWM data or signal data,determines which circuit function is performed as defined in the tablepresented in FIG. 8. That is, as seen in FIG. 8, in order for thecircuit of FIG. 7 to operate in a voltage step up mode, switch M1′ isturned ON, M2 is turned OFF, M3 is modulated with a PWM signal from asuitable duty cycle control circuit (see FIGS. 4 and 5), and both M4 andM5 are turned OFF. Under these conditions, the circuit of FIG. 7effectively reduces to the circuit shown in FIG. 6A, with the onlydifference being diode D3 being added in series with switch M3 (whichaddition does not significantly alter the operation of the circuit). Insuch configuration and mode, the level of the output voltage VOUT isdetermined in large part by the duty cycle of the signal applied to thegate of transistor switch M3, as explained previously.

[0061] Similarly, as defined in FIG. 8, for the circuit of FIG. 7 tooperate in a voltage step down mode, switch M1′ is turned OFF, switch M2is modulated with a PWM signal from a suitable duty cycle controlcircuit 74″ (FIG. 6B), and switches M3, M4 and M5 are all turned OFF.Under these conditions, the circuit of FIG. 7 effectively reduces to thecircuit shown in FIG. 6B, with the only difference being diode D1connected between nodes 72′ and 75′ (which diode does not significantlyalter the circuit's operation), and only a portion of coil L1′ beingused (i.e., only the turns N1 are used). In such configuration and mode,the circuit performs a voltage step down function, as describedpreviously in connection with FIG. 6B.

[0062] As defined in FIG. 8, the circuit of FIG. 7 may also selectivelyoperate in an energy receive mode and a data receive mode by turningswitches M1′, M2 and M3 OFF, and by turning switches M4 and M5 ON. Withthe switches in these positions, the circuit of FIG. 7 effectivelyreduces to the circuit shown in FIG. 6C, and to the circuit shown inFIG. 6D, with the only difference being that just a portion (N1 turns)of the coil L1′ is used as part of the circuit. In this configurationand mode, the circuit of FIG. 7 thus performs both an energy receivefunction as described previously in connection with FIG. 6C, and a datareceive function as described previously in connection with FIG. 6D.

[0063] As further defined in FIG. 8, the circuit of FIG. 7 may alsoselectively operate in a data transmit mode by turning switch M2 OFF, bymodulating switch M3 with a data signal, and by turning switch M1′ ON.Switch M4 may be either OFF or ON depending upon whether capacitor C3′is deemed necessary to better tune coil L1′ for efficient datatransmission. For many data transmissions, capacitor C3′ should not beneeded. Under these conditions, the circuit of FIG. 7 effectivelyreduces to the circuit shown in FIG. 6E. Hence, in such configurationand mode, the circuit performs a data transmit function, as describedpreviously in connection with FIG. 6E.

[0064] Thus, it is seen that by selectively controlling the state of theswitches M1′, M2, M3, M4 and M5, the circuit of FIG. 7 may operate inany one of five different modes. Some of these modes, e.g., the energyreceive mode and the data receive mode, may operate simultaneously.Others of the modes may be invoked in a time-multiplexed manner, e.g.,with a first mode being followed by a second mode, and with the secondmode being followed by a third mode, as required, depending upon theparticular application at hand. Thus, for example, an energy and datareceive mode may operate as a first mode to allow the device to receiveoperating power (e.g, to recharge a battery) and/or to receive initialprogramming control signals. This first mode may then be followed by asecond mode, e.g., a voltage step up mode, initiated by changing thestate of switches M1′, M2, M3, M4 and M5 as defined in FIG. 8, duringwhich the voltage of the primary power source is stepped up to a voltageneeded by the device in order for it to perform its intended function.Subsequently, as required, a third mode, e.g., a data transmit mode, maybe invoked in order to allow the implant device to transmit data to anexternal receiver.

[0065] The component values of the components, i.e., the transistorswitches and capacitors and coil, used in the circuit of FIG. 7 may bereadily ascertained by those of skill in the art for a particularapplication and desired RF frequency.

[0066] It is thus seen that the invention described herein provides avoltage converter circuit for use within an implantable device, e.g.,such as an implantable microstimulator or similar type of neuralstimulator, that is compact, efficient, and provides a wide range ofoutput voltages and currents.

[0067] It is further seen that the invention provides a voltageconverter circuit that avoids the use of a network of capacitorsswitched between parallel and series, or other, configurations in orderto provide the step up and step down voltage conversion function.

[0068] While the invention herein disclosed has been described by meansof specific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. A combination voltage converter circuit forperforming multiple operating functions, the combination circuitcomprising: an RF coil (L1′) having a first end (72′), a second end(85), and a tap point (85′); a diode (Dl) connected between the firstend (72′) of the RF coil (L1′) and an output node (75′); a firstcapacitor (C1) connected between the output node (75′) and groundpotential (87); a first transistor switch (M1′) connected between thesecond end (85) of the coil (L1′) and a primary power source (V_(S)); asecond transistor switch (M2) connected between the tap point (85) andthe primary power source (V_(S)); a second diode (D2) connected betweenthe tap point (85) and ground potential (87); a data amplifier (U1)having an input and an output; a third diode (D3) connected between thefirst end (72′) of the RF coil (L1′) and the input of the data amplifier(U1); a third transistor switch (M3) connected between the input of thedata amplifier (U1) and ground potential (87); a tuning capacitor (C3′)having a first side connected to the first end (72′) of the RF coil(L1′) and a second side connected to the tap point (85′) through afourth transistor switch (M4); and a fifth transistor switch (M5)connected between the tap point (85′) and ground potential (87).
 2. Thecombination circuit of claim 1, wherein the combination circuit isconfigured such that, when transistor switches M2, M4 and M5 are turnedOFF, and transistor switch M1′ is turned ON, and transistor switch M3 ismodulated with a pulse-width modulated (PWM) signal, a voltage isgenerated at the output node (75′) that is stepped up from the voltageof the primary power source (V.); and when transistor switches M1′, M3,M4 and M5 are turned OFF, and transistor switch M2 is modulated with aPWM signal, a voltage is generated at the output node (75′) that isstepped down from Vs; and when transistor switches M1′, M2 and M3 areturned OFF, and transistor switches M4 and M5 are turned ON, thecombination circuit operates in an energy receive mode wherein RF energyreceived through the RF coil (L1′) is rectified and stored in capacitorC1, and also operates in a data receive mode wherein data modulating thereceived RF energy is demodulated through diode D3 and recoverable asdata at the output of amplifier U1; and when transistor switches M1′, M4and M5 are turned ON, transistor switch M2 is turned OFF, and transistorswitch M3 is modulated with data, the combination circuit operates in adata transmit mode wherein the data modulating transistor switch M3 istransmitted from the RF coil (L1′).
 3. The combination circuit of claim2, wherein the combination circuit is configured for use in animplantable medical device.
 4. The combination circuit of claim 3,wherein the tap point (85′) on the RF coil (L1′) divides the coil intoN1 turns between the tap point and the first end (72′) of the coil, andN2 turns between the tap point and the second end (85) of the coil, andwherein the inductance value of the RF coil is between 1 μH and 1000 μH.5. The combination circuit of claim 3, wherein the number of turns N1 isbetween about 1 and 1000 turns and the number of turns N2 is betweenabout 1 and 1000 turns.
 6. A combination voltage converter circuit forperforming multiple operating functions, the combination circuitcomprising: a coil designated as L1′ having a first end (72′), a secondend (85), and a tap point (85′), wherein the tap point (85′) on the RFcoil (L1′) divides the coil into N1 turns between the tap point and thefirst end (72′) of the coil (L1′), and N2 turns between the tap pointand the second end (85) of the coil; and circuit means, includingswitches, for implementing operating modes and switching betweenoperating modes of the combination circuit, wherein the operating modesinclude (i) power receive inductively through the coil (L1′), (ii)voltage step up conversion using the coil (L1′) and (iii) voltage stepdown conversion using the coil (L1′); and wherein the coil is coupled tothe circuit means.
 7. The combination circuit of claim 6, wherein thenumber of turns N1 is between about 1 and 1000 turns and the number ofturns N2 is between about 1 and 1000 turns; and wherein the inductancevalue of the coil (L1′) is between about 1 pH and 1000 pH.
 8. Thecombination circuit of claim 7, wherein, in the voltage step downconversion operating mode, only the N1 part of the coil L1′ is used. 9.The combination circuit of claim 6, wherein the circuit means and thecoil are configured to receive data or commands via the coil concurrentto receiving power inductively; and wherein the operating modes includefour modes: (i) power receive inductively through the coil and,concurrently, data receive through the coil (ii) voltage step upconversion using the coil (iii) voltage step down conversion using thecoil and (iv) data transmit through the coil.
 10. The combinationcircuit of claim 9, wherein the switching means comprises first (M1′),second (M2), third (M3), fourth (M4) and fifth (M5) transistor switches,which switches are selectably turned on, turned off, or modulated invarious switch combinations to configure the combination circuit to atleast one of the four operating modes; and wherein the selection of theoperating modes is implemented through a time multiplexing scheme. 11.The combination circuit of claim 10, wherein the circuit means includesa pulsewidth modulation (PWM) circuit for controlling one of thetransistor switches, which PWM circuit is used for voltage step upconversion or voltage step down conversion.
 12. The combination circuitof claim 10, wherein the circuit means includes an ON/OFF modulation(OOM) low power circuit for controlling one of the transistor switches,which OOM circuit is used for voltage step up conversion or voltage stepdown conversion.
 13. The combination circuit of claim 10, wherein thenumber of turns N1 is between about 1 and 1000 turns and the number ofturns N2 is between about 1 and 1000 turns; and wherein the inductancevalue of the coil (L1′) is between about 1 pH and 1000 pH.
 14. Thecombination circuit of claim 13, wherein, in the voltage step downconversion operating mode, only the N1 part of the coil L1′ is used. 15.The combination circuit of claim 13, wherein, in the energy receive anddata receive operating modes, only the N1 part of the coil L1′ is used.16. A method for using a single, combination voltage converter circuitto perform multiple operating functions in an implantable medicaldevice, the method comprising: (a) providing an electronic circuitry,including a coil, which electronic circuitry is incorporated in theimplantable device, wherein the electronic circuitry includes aplurality of switches; (b) selecting one of the operating modes of thecombination circuit, wherein the possible operating modes of thecombination circuit include: (i) power receive and, concurrently, datareceive using the coil, (ii) data transmit using the coil and (iii)voltage step up conversion using the coil, and (iv) voltage step downconversion using the coil; and (c) repeating the step (b) above as manytimes as necessary to provide a desired sequence of operating modes, andthereby implementing each operating mode in a time-multiplexed scheme.17. The method of claim 16, wherein the step (b) selecting one of theoperating modes of the combination circuit is accomplished by usingtransistor switches, which transistor switches comprise a first (M1′),second (M2), third (M3), fourth (M4) and fifth (M5) transistor switches;and wherein the switches are turned on, turned off, or modulated invarious switch combinations to configure the combination circuit to atleast one of the four operating modes.
 18. The method of claim 17,wherein the step (b) selecting the operating mode for (iii) the step upvoltage conversion or for (iv) the step down voltage conversion isaccomplished by applying modulation to one of the transistors; andwherein modulation is produced at the modulated transistor from apulse-width modulated (PWM) circuit or an ON-Off modulation (OOM) lowpower modulation circuit.
 19. The method of claim 17, wherein the step(b) selecting the operating mode for (i) of the combination circuit forthe power receive and data receive mode is accomplished by receivingalternating RF energy signals through the RF coil (L1′), rectifying theenergy signals and storing the energy in capacitor C1 and, concurrently,data modulating the received RF energy signals, demodulating the signalsthrough a diode D3 and recovering data at the output of amplifier U1;and wherein the transistor switches M1′, M2 and M3 are turned OFF andtransistor switches M4 and M5 are turned ON.
 20. The method of claim 17,wherein the step (b) selecting the operating mode of the combinationcircuit for (iv) the data transmit mode is accomplished by turning ONthe transistor switches M1′, M4 and M5, turning OFF transistor switchM2, and modulating the transistor switch M3 to modulate data which istransmitted from the coil (L1′).